Clean and de-vain crayfish then remove tails. Cut into chunks. Set legs aside.
In a wok, heat oil then stir fry onion, garlic, lemongrass, ginger, coriander, shallot and chillies. Cook until shallot and onion become clear (add water if needed). Sir in black beans and crayfish.
Raise heat to high and stir fry until shells have changed colour, 5-10 minutes. Stir in soy sauce then cover and cook until done, about 5 minutes. Stir in lime juice, lemon juice and coriander then serve over rice.